Radio frequency amplifier



Jan. 17, 1933. Y. WALSH 1,894,503

$111310 FREQUENCY AIPLIFIER Filed Sept. 26, 1927 5 Sheets-Sheet 1INVENTOR z/mzn Walsh ATTORNEYS Jan.17,1933. AL 1,894,503

RADIO FREQUENCY AMPLIFIER Filed'Sept. 2a. 1927 5 sheets-sheet 2 INVENTOR2' (:2 5m lf a 25/1 ATTORNEY$ Jan. 17, 1933. I L. WALSH nmxo FREQUENCYAMPLIFIER Filed Sept. 26. 1927 5 Sheets-Sheet 4 ATTORNEYS Jan 17, 1933.L. WALSH 1,394,503

RADIO FREQUENCY AMPLIFIER Filed Sept. 26. 1927 5 Sheets-Sheet 5 W0ve/e/rgfb med: r

eco 5 o I .500 Fhyuengf- /7//0 qyc/es INVENTOR LII/coin Walsh ATTORNEY 5Patented Jan. 17, 1933 UNITED STATES PATENT OFFICE v I LINCOLN WALSH, OFELIZABETH, NEW JERSEY, ASSIGNOR TO HAZELTINE CORPORA- TION, OF JERSEYCITY, NEW JERSEY, A CORPORATION OF DELAWARE RADIO FREQUENCY AMPLIFIERApplication filed September 26, 1927, Serial No. 222,009, and in CanadaDecember 14, 1927.

The present invention relates to radio-frequency vacuum tube amplifiersof the transformer-coupled type and, more particularly, tuned amplifiersintended to operate over a fairly wide range of frequencies, as forexample, in broadcast receivers.

In the design of tuned radio-frequency amplifiers one of the controllingfactors to be dealt with is the tendency toward excessive regenerationand consequent oscillation which is due mainly to the coupling capacitybetween the grid and plate of each vacuum tube. Some provision mustalways be made against this tendency, preferably by neutralization. Butsince neutralization involves the use of a fixed neutralizing capacitywhich is balanced against the coupling capacity of a "acuum tube andsince individual vacuum tubes, even of the same type and manufacture,are bound to vary to a certain extent, it follows as a direct resultthat there is a limit to the amplification per stage at any givenfrequency beyond which it is impracticable to go. Since theinter-electrode (plate-grid) capacitive reactance decreases with risingfrequency, thus allowing a greater amount of high-frequency energy to befed back from the plate circuit to the grid circuit of each tube, thepermissible amplification per stage also decreases with risingfrequency. As a matter of experience, the permissible amplification perstage at 1500 kilocycles per second is very much less than at 500kilocycles per second. These fre quencies are the limits of thebroadcasting band as now established. Theory and experience bothindicate that the amplification should vary inversely as the square rootof the frequency in order that adequate manufacturing tolerance in thevacuum tube coupling capacity may be allowed.

On the other hand, the amplification per stage obtainable with couplingtransformers of the conventional type, that is, transformers havingfixed coils of high stepup ratio and low losses, is less atthe iowerthan at the higher frequencies. is between the frequent-cs 500kilocycles per second and 1500 kilocyclcs per second the actuala1nplitication per stage is very much less at the lower frequency. Thus,while the permissible amplification perstage at the lowerfrequency endof the broadcast band is much greater than at the high frequency end,the characteristics of the conventional radiofrequency transformer aresuch that the actual amplification obtainable is very much less at thelow frequencies than at the high frequencies.

It has sometimes been attempted to modify the amplificationcharacteristics of the conventional transformer to increase theamplification at low frequencies without also increasing theamplification at high frequencies. This can be done by introducingsources of energy loss which are mainly effective at high frequencies,but this expedient has the serious disadvantage of impairing theselectivity at high frequencies.

Notonly is the lower-frequency amplification with conventionaltransformers limited by the instability which would occur at higherfrequencies, but the conventional transformer arrangement does notpermit of high amplification at low frequencies'irresoective ofinstability. The reason is that in order to be able to tune to allfrequencies within the broadcast band it is necessary to employ a tuningcondenser having relatively great capacity at the lower frequencies.Such a large capacity in combination with transformer coils ofconventional small dimensions and such as will give satisfactoryfidelity, introduces an excessive secondary conductance at lowfrequencies. The high secondary conductance thus introduced has theeffect of reducing the amplification at low frequencies, as may beeasily demonstrated both mathematically and by actual performance tests.

, The primary object of the present invention is to provide aradio-frequency amplifier from which may be obtained, at all frequencieswithin its range, the highest amplification consistent with stability,as limited by the customary tolerance in coupling capacities of vacuumtubes, without impairing the selectivity at high frequencies where thisis naturally thepoorest), or the fidelity at low frequencies (where thelatter characteristic is naturally the poorest).

Another object, in furtherance of the primary invention, is to provide astructural and electrical arrangement which is adequately simple andtrouble-proof in operation. The present invention accomplishes theseends by providing means adapted to change vacuum tube and whosesecondary winding together with a tuning condenser is connected in thegrid-filament (input) circuit of a sueceeding vacuum tube, the greatestamplification would occur at the highest frequency to which the receivermay be tuned, and the least amplification at the lowest frequency.

Therefore, it is ordinarily necessary to design such transformer so asto avoid exceeding the permissible amplification per stage at thehighest frequency, as limited by the tendency toward instability.

In so far as the transformer per se is concerned, the factors'whichordinarily determine the degree of amplification are the selfinductanceof the secondary winding, the mutual inductance between primary andsecondary winding and the conductance (or resistance) of the secondarywinding. The voltage ratio, or simply the ratio of a transformer thesecondary circuit of which is tuned is substantially equal to the ratioof the secondary self-inductance to the mutual inductance. In theconventional radio-frequency transformer now referred to, both thesecondary self-inductance and the mutual inductance remain constantirrespective of the frequency, and these values must, therefore, bedetermined by the requirement. that the maximum permissibleamplification must'not be exceeded at the highest operating frequency.If the ratio of secondary self-inductance to the mutual inductance islowered as the operating frequency is lowered then desired amplificationwhile maintaining fi-' delity and compactness of coil construction.

the amplification per stage will in general be increased. Furthermore,if the secondary self-inductance is increased as the frequency islowered, it will be possible to obtain the The present inventioninvolves the proper adjustment of the inductanees and the tuningcapacity so as to obtain the maximum permissible amplification at eachfrequency within the band of frequencies for which the receiver isdesigned.

Theoretically there are a variety of ways be called, are preferably madeof a non-magnetic metal of high conductivity, such as copper oraluminum. Each cup is arranged to slide longitudinally with respect toits associated transformer. The larger cup is adapted to variablyenvelop the outer winding, which is the secondary, while the smaller cupis adapted to move longitudinally within the smaller or primary winding.The effect of moving the metallic shields or cups with respect to thetransformer windings is to vary both the secondary self-inductance andthe mutual inductance. each at the desired rate. The effect of the metalcups or shields disposed closely adjacent the windings of thetransformers is to restrict the cross section and thereby to increasethe reluctances of the magnetic paths, thus reducing the inductances.

The transformers may accordingly be designcd with a view to obtainingthe maximum permissible amplification per stage at the lowest operatingfrequencythis value being much higher than that of the permissibleamplification at the highest operating frequency. Then by providingmeans for moving the metal shields or cups so as to increasingly envelopthe transformer windings as the operating frequency is increased theamplification may be decreased to the proper value for each operatingfrequency.

In the preferred form of this invention the movement of the metal shieldor shields relatively to the transformer windings is accomplishedautomatically by means of suitable mcchanical connections with thetuning control or controls, and preferably the several stages ofamplification are adjusted simultaneously by a single manual control.However, some of the advantages of the invention together with greaterflexibility may be obtained by independently operating the shields andthe tuning condensers.

With reference to the drawings which accompany this specification,

Fig. l is a cross sectional view of a coupling unit comprising aradio-frequency transformer with its associated metal shield togetherwith a variable tuning condenser and a neutralizing condenser, all ofWlllCll are assembled within a sheet metal receptacle; n

Fig. 2 is an elevational view of the coupling unit shown in-Fig. 1 andis taken as i/ iewed from the same observation point as Fig. 3 is apartial elevational view taken along the line 3-3 of Fig. 1 and lookingtoward the front of a radio receiver. This figure illustrates theoperating mechanism for the. shields and tuning condensers;

Fig. at is a sectional view taken along the line 44 of Fig. 1;

Fig. 5 is a plan view of a condenser plate;

Fig. 6 is a comparative amplification graph representing by appropriatecurves the amplification obtainable at different frequencies with aconventional tuned amplifier and with an amplifier in accordance withthis invention; and

Fig. 7 is a partial circuit diagram of a neutralized radio-frequencyamplifier adapted for use in conjunction with this invention.

In Figs. 1 and 2 there are illustrated in section and elevation,respectively, a radiofrequeucy transforn'ler with its associatedmetallic cup-like shields together with a tuning condenser and aneutralizing condenser-the arrangement shown being in accordance withone of the preferred embodiments of the invention. The transformer perse comprises two coaxial tubes 1 and 2 of dielectric material such asformica. The tube 1 is mounted inside the tube 2, being spaced therefromby means of a suitable spacing ring 3 and secured by bolts 4. In thisparticular case there is, in addition to the prij mary winding, aneutralizing winding having an equal number of turns, wound on thetube 1. The turns of the primary and neutralizing windings areinterleaved. The secondary is wound on the tube 2. The metal-- lieshield 5 comprises two coaxial cylindrical cup-like members 6 and 7. Thetwo cups are secured together forming a unit. The tuning condensercomprises two identical elements, namely, a stator 8 and a movableelement 9. Each of these elements has four plates, as shown. Theplates'of one element are interleaved with those of theother. Themovable element 9 is rigidly secured to the shield 5 through the mediumof a connecting member 10 of insulating material. The shield 5 togetherwith the movable condenser element 9 is adapted to slide axially withrespect to the transformer. Thus it will be seen that as the plates ofthe tuning condenser move togetherthe capacity of the condenser therebyincreasing-the shield 5 is moved to the right. as shown in Fig. 2,whereby it decreasingly affects the magnetic field of the transformercausing both the secondary self-inductance and the mutual inductance toincrease. lVhen the cup '6 envelops the secondary winding to the maximumextent the reluctance of the magnetic field is a maximum, and thesecondary selfinductance is consequently reduced to a minimum. Likewisethe mutual inductance between primary and secondary windings isdecreased as the cup 7 is moved into closer relation with the primarywinding, that is, to the left as viewed in Fig. 1. While the two shieldsinteract in their effects the outer one 6 aflects mainly the secondaryself-inductance, and the inner one 7 the mutual inductance. lVith theproportions indicated'in Fig. 1of which specific dimensions will begiven-the mutual inductance varies at a faster rate than the secondaryself-inductance, giving the desired variation in the ratio. Although thearrangement illustrated comprising two coaxial cups constitutes thepreferred embodiment of the invention, it has been found that similarbut less ideal results may be obtained by omitting either one or theother of the cups 6 and 7 Since the function of the shields is to varythe reluctance of .the magnetic field it will be apparent that theshield or shields may take a variety of forms besides the specificarrangement illustrated.

As a part of the tuning condenser structure illustrated in Figs. 1 and 2there is shown a flexible conducting element 11 which is adapted tofunction as one plate of a neutralizing condenser. This flexible platemay be adjusted toward and away from the adjacent fixed plate of thestator 8 by means of the adjusting screw 12. The capacity between theelement 11 and the stator of the tuning condenser may thus be adjustedto a proper value for efiecting neutralization.

In order to clarify the illustration, the operating mechanism for thetwo tuning condensers and shields has been omitted from Figs. 1 and 2. Asuitable mechanism for this purpose is illustrated in Figs. 3 and 4. Itwill be realized, however, that the operating mechanism illustrated inFigs. 3 and 4 constitutes only one of a great variety of ways in whichthe same results may be obtained. and that the invention is in nowisedependent upon the particular mechanism by which the operation of thetuning condensers and shields is brought about.

The operating mechanism of Figs. 3 and 4 is arranged for the unitaryoperation of two sets of tuning condensers and shields. It mayobviously-be extended to take care of as many stages of amplification asmay be desired.

Figs. 3 and 4; are views taken along the lines 33 and 4-4, respectively,of Fig. 1

and drawn to a somewhat smaller scale than- Fig. 1. As previouslystated, the operating mechanism shown in Figs. 3 and 4, and

meshes with a toothed quadrant carried by a shaft 16. A graduated dial15a is attached to and moves withthe quadrant 15. A leyer arm 17 whichfor convenience is made in the form of an L, is secured to shaft ,16 andis rotatable therewith. Tov the lever 17 is pivotally secured a link 18which is connected to the member 10 (shown also in Figs. 1 and 2). Sinceboth the shield 5 and movable condenser element 9 are attached to themember 10, it is apparent that these parts all move together in adirection parallel to the axis of the transformer in response torotation of the tuning control knob. As shown and shields indicated inFig. 4 are operated from the same control knob, and that as manyadditional sets of condensers and shields as might be desired could beadded and operated through the one control. Three stages of tunedradio-frequenc amplification have been found very suita le.

Incidentally, the present invention lends itself very advantageously tounitary tuning control. tuning condensers are preferabl of small maximumcapacity as compared with thetuning condensers ordinarily used inbroadcast receivers of the conventional type. The reason wh aparticularly small tuning condenser can e used to cover a wide band offrequencies is that the tuning is accomplished by varying the secondaryself-inductance at the same time that the tuning capacity is varied.Since the tuning condensers may be of unusually small maximum capacitythe plates may be heavier and spaced farther apart without excessivebulkincss. For these reasons it is possible to maintain a consid erablygreater precision in manufacturin the condensers; and the tuning of thesevera stages of amplification under a single control may, therefore, becarried out with greater accuracy or with less difliculty.

As a specific example, the dimensions of the transformer, shield andtuning condenser shown in Figs. 1 and 2 will now be given. The tube 1 ismade of natural formica 1% inch outside diameter, 3 inches long, and1/32 inch thick. On this tube are wound a primary coil and aneutralizing coil. The turns of these coils are interleaved, each coilhaving 36 turns of number 38 double-silkcovered copper wire, 16 doubleturns per inch.

This is because of the fact that the tween about 550 kilocycles The tube2 is made of the same material, 2 inches outside diameter, 3% incheslong and 1/16 inch thick. On this tube is wound a secondary coilconsisting of 120 turns of number 26 enameled copper wire, 48 turnsperinch. The cup 6 which forms part of the metal shield is made of sheetcopper 0.031 inch thick. Referring to Fig. 1, dimension A is 2 11/16inches and B is 2.95 inches. Cup 7 is also made of sheet copper 0.031inch thick. Dimension C is 2 inches and D is 1% inches.

With the above dimensions, the step-up ratio of the transformer is atall frequencies substantially higher than that giving greatestamplification; or, in other words,the input conductance at resonance issubstantially higher than the plate conductance of the preceding vacuumtube. This is for the purpose of increasing selectivity and stability,

as explained in the co-pending application of Louis A. Hazeltine',Serial No. 12,000, filed February 27, 1925.

- The tuning condenser elements 8 and 9 are identical-one being fixedand the other movable; Each consists of four aluminum plates 0.031 inchthick and spaced 0.187 inch apart. A plan view of one condenser plate isshown in i 5. Suitable values for the dimensions indicated in Fig. 5 areas follows:

E 3 inches F 2% inches G 1 inch H 1 inch J inch K= inch This particularcondenser was designed with a view to obtain tunmg characteristics lyingapproximately midway between- 1 and 2 employed in a multistage ampli erusing vacuum tubes of the 201A type having an amplification factor ofabout 8, the results actually obtained are indicated by curve B ofFig.6., On this graph sheet the curve A shows the calculated maximumpermissible amplification at all frequencies be- Ker second and 1550kilocycles per second. ccordin g to the theory on which curve A isbased, the amplification should vary inversely as the square root of thefrequency in order that a practical manufacturing tolerance may beallowed with respect to the plate-grid coupling capacity. This is wellconfirmed by actual experience. It will be observed that curve B closelyfollows the form of curve A and,

therefore, indicates an amplification which varies inversely as thesquare root of the frequency. In calculating the permissibleamplification at the various frequencies from which curve A was plotteda tube capacity tolerance of 0.5 micromicrotarad was allowed. Thistolerance represents the permissible manufacturing deviation of theinternal capacity between the plate and grid. It is this unavoidabledeviation which exercises the greatest influence in limiting thepermissible amplification. Curve C, Fig. 6, indicates theradio-frequency amplification obtainable in a good representativeneutralized receiver having radio-frequency transformers of the generalform shown in Fig. l but without the metallic shields or equivalentmeans for accomplishing the same purpose. From an examination of curve(J it will be at once apparent that the amplification at thehighfrequency end of the band is much greater than at the low-frequencyend. Also by comparison of curve C with curve A it will be seen that theamplification at the low-frequency end is very much less than thepermissible amplification, whereas it closely approaches the maximumpermissible amplification at the high-frequency end.

Curve B indicates how the large discrepancy between permissibleamplification and actual amplification at low frequencies and, in fact,over substantially the entire band of frequencies except for theimmediate region of the upper end, has been corrected by the use ofmetal shields as herein described.

The specific structure which has been described as an examplerepresentative of the preferred embodiment of the invention isapplicable for use in the radio-frequency portion of a neutralizedbroadcast receiver of which a partial circuit is shown diagrammaticallyin Fig. 7 This figure illustrates diagrammaticallytwo stages ofneutralized ra (ho-frequency amplification the output of which may bepassed through one or more additional stages of radio-frequencyamplification and thence to adetector or it may be passed directly tothe detector and thence to an audio-frequency amplifier.

In Fig. 7 the three-electrode vacuum tube amplifiers 20 and 21,respectively, are coupled in cascade through the medium of aradio-frequency transformer '22 which may be the transformer shown inFigs. 1 and 2. The primary Winding 23 and the neutralizing winding 24 ofthis transformer are interleaved on the same tube, as-previouslydescribed in connection with Fig. 1, and may be in allrespects-identical. One end of the neutralizing coil 24 is connected toa neutralizing condenser 25 the other terminal of which is connected tothe grid of the tube 20. This is in accordance with one form of the wellknown Hazeltine method of neutralization. The secondary winding 26 ofthe transformer 22, Fig. 7, may be the same as specified hereinbefore inconnection with the description of Fig. 1. The tuning condenser 27 maybe identical with that shown in Figs.

1, 2 and 5. The transformer 28 which couples the output side of tube 21with the next succeeding tube (not shown) may be identical withtransformer 22 and, of course, the tuning condenser 29 may be identicalwith the condenser 27. Likewise the neutralizing condenser 30 may be inaccordance with the disclosure of Fig. 1 wherein the flexible element 11comprises one plate of a neutralizing condenser of which the stator ofthe tuning condenser forms the other plate.

The present invention has been developed in conjunction with neutralizedamplifiers of the type indicated in general by Big. 7 and isparticularly well adapted for use in connection with that type ofneutralization, but it may be used effectively with other methods ofneutralization and oscillation suppression.

What is claimed is:

1. In a vacuum tube amplifier, a plurality of vacuum tubes, a tunablecoupling means interconnecting the output side of one of said tubes withthe input side of another of said tubes, said coupling means comprisinga transformer having a primary winding and a secondary winding and avariable tuning condenser, and means operable to vary, simultaneously,the tuning capacity, the secondary self-inductance, the mutualinductance and the ratio of secondary to mutual inductance of saidtransformer whereby the degree of amplification may be increased as thefrequency to which the amplifier is tuned is decreased.

2. In a vacuum tube amplifier comprising a plurality of vacuum tubes, acoupling transformer interconnecting the output side of one ofsaid tubeswith the input side of another of said tubes, said transformercomprising a primary winding and a secondary winding, a variable tuningcondenser connected across said secondary winding, and means operableconjointly with said condenser to decrease the self-inductance of saidprimary winding and of said secondary Winding and decrease the mutualinductance coupling system is tuned is decreased andvice-versa.

4; A multistage vacuum tube amplifier comprising a pair of vacuum tubes,a coupling'transformer interconnecting the output side of one of saidvacuum tubes withthe input side of the other of said vacuum tubes, saidtransformer comprising a primary winding and a secondary winding, avariable tuning condenser connected across said secondary winding, saidamplifier being intended to selectively amplify waves of all frequencieswithin a predetermined band of substantial width, said primary andsecondar windings being so designed that when said condenser is adjustedto its maximum capacity, theresultant amplification will at leastapproximate the maximum permissible amplification as determined by thelimit of stability, and a shield adjustable in position relative to saidtransformer and operable to variably restrict the magnetic path of saidtransformer, said shield being increasingly effective to decrease theself-inductance of said secondary winding and the mutual inductance ofsaid transformer as it is moved into increasingly intimate relation tosaid windings.

5. A multistage vacuum tube amplifier comprising at least two vacuumtubes, a coupling transformer interconnecting the output side of one ofsaid vacuum tubes with the input side of another of-said vacuum tubes,said transformer comprising a primary winding and a secondary winding, avariable tuning condenser connected across said secondary winding, saidamplifier being intended to selectively amplify waves of all frequencieswithin a predetremined band of substantial width, said primary andsecondary windings being so designed that when said conenser is adjustedto its maximum capacity the resultant amplification will at leastapproximate the maximum permissible amplification as determined by thelimit of stability and an adjustable shield operable to variablyrestrict the magnetic path of said transformer,,said shield beingincreasingly effective to decrease the self-inductance of said secondarywinding and the mutual inductance of said transformer as it is movedinto increasingly intimate relation to said windings, an driving meansoperable to adjust the position of said shield conjointly with tuningadjustments of said condenser, whereby the secondary self-inductance andthe mutual inductance of said transformer are maintained at'such valuesas will result in at least an approximation to the maximum input side ofthe other of said tubes, sail I coupling system comprising a highfrequency transformer having a primary winding and a secondary winding,a variable tuning condenser connected in circuit with said secondarywinding, a metal shield operatively associated with said transformer,said shield and said transformer being movable relative to each other,said shield being operable to vary the self-inductance of said secondarywinding and the mutual inductance between said windings and also to varythe ratio of said self-inductance to said mutual inductance, saidtransformer and tuning condenser being designed to provide high amplification with stability at low frequencies, and means for bringing saidshield into increasingly intimate relation with said transformer as theoperating frequency is increased whereby'the amplification is decreasedwith increasing frequency and'thereby maintained within the limit ofstability at all frequencies.

7. A radio-frequency vacuum tube amplifier comprising a plurality ofvacuum tubes, the output terminals of each tube being coupled with theinput terminal of the succeeding tube through the medium of a tunablecoupling system, means for neuiralizing the capacity coupling of each ofsaid tubes. each of said coupllng systems comprising a transformerhaving a primary winding and a secondary winding, a variable tuningcondenser connected in circuit with said secondary winding, saidtransformer being designed to permit of obtaining at least anapproximation to the maximum permissible amplification at the lowestfrequency to be amplified, a metal shield operable to restrict themagnetic path of said transformer to a variable extent dependent uponits position with respect to said transformer and means operable toadjust said tuning condenser and said shield conjointly whereby themagnetic path of said transformer is increasingly restricted as theeffective capacity of said luning condenser is increased, thearrangement being such that. the secondary self-inductanee and themutual inductance of said transformer are both reduced, as the frequencyis increased, so that the amplification at each frequency is near to butnot in excess of the maximum permissible amplification at thatfrequency.

8. In a vacuum tube coupling system, a radio-frequency transformercomprising a primary winding and a secondary winding, a tuning condenserconnected in circuit with one of said windings, a conductive shieldadapted to variably restrict the magnetic path of said transformer, andmeans operable to conjointly vary the capacity of said condenser and theposition of said shield relative to said transformer.

9. A vacuum tube coupling system, comprising a transformer having aprimary winding and a secondary Winding, a tuning condenser connected incircuit with said transformer, and a metallic shield for saidtransformer, said shield comprising two cuplike members, one of which ismore particularly assocated with said secondary winding, said membersbeing operative to vary the mutual inductance and the secondaryselfinductance of said transformer, respectively, and driving means foradjusting the position of said shield relative to said transformer andthe capacity of said condenser conjointly.

10. In a radio-frequency amplifier, a plurality of vacuum tubes and aplurality of tunable coupling systems interconnecting said vacuum tubes,each of said coupling systems comprising a transformer, a variabletuning condenser and a shield operable to vary the electricalcharacteristics of said transformer, and control meansfor-simultaneously varying the capacities of said condensers and thepositions of said shields relative to said transformers.

11. The combination with a high-frequency transformer comprising aprimary winding and a secondary winding of a variable condenser incircuit with said secondary winding, a metallic shield comprising twoco-axial cup-like members, said shield being movable axially withrespect to said windings and operable to variably restrict the magneticpaths thereof, and a mechanism for operating said condenser and saidshield conjointly whereby said magnetic paths are restricted to thedesired extent for each operating position of said condenser.

12. In a stage of tuned radio-frequency amplification employing a tuningsystem coupling two vacuum tubes said system including inductance andcapacity, with means for neutralizing the capacity coupling of at leastone of said vacuum tubes, the method of attaining high amplification,high selectivity and freedom from oscillations at all radiofrequencicswithin a wide range, with a single tuning control'nieans, which methodcomprises simultaneously decreasing the selfinductance of the tuninginductance, decreasing the tuning capacity and raising the stepupvoltage ratio as the frequency is raised, at such respective rates thatthe voltage amplification varies approximately inversely as the squareroot of the frequency, whereby the limiting permissible variation ofsaid coupling capacity is approximately the same at all frequencies.

13. The method of improving the effectiveness of a transformer-coupledhigh-frequency vacuum tube amplifier having a tuning condenser incircuit with the secondary winding of the coupling transformer, saidmethod comprising :-automatically progressively decreasing the primarysclf-inductanc e, the secondary self-inductance and the mutualinductance of said transformer as the effective capacity of the tuningcondenser is decreased, and progressively increasing the secondaryself-inductance and the mutual inductance as the effective capacity ofthe turn ing condenser is increased.

14. The method of operating a tuned radio frequency vacuum tubeamplifier having a tuning capacity, wherein vacuum tubes areinterconnected through the medium of transformers, said methodconslsting 1n ncreasing the reluctance of the magnetic path of thetransformer as the tuning capacity is decreased.

15. In the operation of a vacuum tube amplifier of the tunedtransformer-coupled type, the method of increasing the amplification athe lower frequencies witho'ut impairing the selectivity at the higherfrequencies and with out exceeding the limit of stability at anyfrequency, said method consisting in automatically varying the primaryself-inductance, the secondary self-inductance and the mutual inductanceof the transformer simultaneous 151 while tuning.

16. In a radio-frequency coupling system an inductance coil, means fortuning said system over a relatively wide range of frequencies, aconductive shield comprising a cuplike member, said shield being movableaxially with respect to said coil and adapted to variably restrict themagnetic paths thereof, and means operable to vary the position of saidshield conjointly with said tuning means.

17. In a radio frequency coupling system, an inductance coil. means fortuning said system over a relatively wide range of frequencies, aconductive shield comprising a plurality of cup-like members, saidshield being movahle axially with respect to said coil and adapted tovariably restrict the magnetic paths thereof, and means operable to varythe position of said shield conjointly with said tuning means.

18. In a vacuum tube amplifier system ineluding a radio-frequencycoupling inductance, means for tuning said system over a relatively widerange of frequencies, wherein a voltage built up across said inductancetends to produce oscillations at the higher frequencies of said range, aconductive shield positioned in the magnetic paths of said inductanceand movable relatively thereto, and means for effecting relativemovement between said shield and said inductance and interrupting saidpaths to decrease said voltage proportionately as said system is tunedto higher frequencies.

19. In a vacuum tube amplifier system including a radio-frequencycoupling inductance, means for tunlng said system over a relatively widerange of frequencies, whereln voltage built up across said inductancened to higher frequencies. 20. In a Vacuum tube amplifier systemincluding a radio-frequency coupling inductance, means for tuning saidsystem over a relatively wide range of frequencies, wherein voltagebuilt up across said inductance tends to produce oscillations at thehigher frequencies of said range, a conductive shield positioned inthemagnetic paths of said inductance and movable relatively thereto, andmeans for effecting relative movement between said shield and saidinductance and interrupting said paths to decrease said oscillationproducing voltage automatically as without impairing the selectivityatid system is tuned to higher frequencies. In testimony whereof I aflixmy signature.

LINCOLN WALSH.

place-AMER 1,894,503r-I1i7g00ln I I dated'January 17, 1933.. vDisclaimer filed May 29, 1934,-by the patentee, 7 the assignee,]Ha;zeltine Corporation, consenting. I Hereby 'enters this disclaimerofclaim 15 in said specification, which is in the following words, towit:

f15. In the operation of a vacuum tube amplifier-of the tunedtransformercoupled type,-,. the method of increasing-the amplificationat the lower frequencies the higher frequencies and without exceedingthe limit of stabilit at any frequency, said; method consisting inautomaticallyvarymg the primary se -inductanc.e, the secondaryself-inductance and the mutual mductanoe ofthe transformersimultaneously whiletuning.

- [Qfidiul Gazette June19, 1934.]

Walsh,oElizabeth, Nj. J. RAmo Fnnounnc rifittrmrina; "Patent'

